1. Field of the Invention
The invention relates to a process for the preparation of iminodisuccinic acid ammonium metal salts by reaction of maleic anhydride, alkali metal hydroxides, ammonia and water in a first stage to give iminodisuccinic acid ammonium salts and their subsequent reaction with metal oxides, metal hydroxides or other metal salts in a second stage. The products resulting therefrom can be used to increase the availability of metal ions, e.g. in agriculture, as trace element fertilizers or slug and snail pellets, or in the ceramics industry, for surface colouring.
2. Brief Description of the Prior Art
Trace element fertilizers are used in agriculture to increase agricultural yields and to prevent plant diseases, such as apple spot (brown spots) or tomato fruiting rot (black spots) which can be traced back to a disrupted supply of calcium. Illustratively, there is employed specific calcium, magnesium, manganese, iron, copper or zinc salts of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) or other complexing agents.
EDTA and DTPA are two conventional complexing agents which have been used for years in large amounts. However, many of the conventional complexing agents, such as EDTA and DTPA and various phosphonates and the metal complexes resulting therefrom, are not at all or only partially biodegradable; they remobilize heavy metals in surface waters and can even enter drinking water treatment, since they are not adsorbed in sewage sludges or soils.
It is therefore an important object to develop complexing agents which do not have the eco-toxicological disadvantages, such as described above.
Iminodisuccinic acid is an art-known complexing agent which is readily biodegradable and which therefore has an eco-toxicological advantage with respect to the complexing agents. Its metal salts, which can be used in agriculture and in the ceramics industry, are also biodegradable, and the preparation process is environmentally friendly, since it generates no significant solid or liquid waste and any ammonia released can be recycled in chemical processes.
GB 1 306 331 discloses the preparation of iminodisuccinic acid from maleic acid and ammonia in a molar ratio of 2:3 to 2:5 at temperatures of 60 to 155° C. For their work up, either hydrochloric acid or sodium hydroxide solution can be added. In SU 0 639 863, iminodisuccinic acid is prepared in the presence of alkali metal hydroxides from maleic acid and ammonia at a molar ratio of 2:0.8 to 2:1 at temperatures of 110 to 130° C. JP 6/329 606 discloses a three-stage process for the preparation of iminodisuccinic acid. A maleic acid derivative is first reacted with ammonia in an aqueous medium. Alkali metal or alkaline earth metal hydroxides are then added. A “maturing process” follows in the third process stage. JP 6/329 607 likewise discloses a three-stage process for the preparation of iminodisuccinic acid. In the first stage, a maleic acid derivative is again first reacted with ammonia in aqueous medium. Alkali metal or alkaline earth metal hydroxides are then added in the second stage. In the third stage, the reaction is continued after addition of further maleic acid derivative.
However, in none of the patents is a process for the preparation and the use of the iminodisuccinic acid ammonium metal salts according to the invention disclosed.
A process for the preparation of iminodisuccinate alkali metal salts and their suitability as complexing agents are known from U.S. Pat. No. 6,107,518. It is known, from U.S. Ser. No. 2001/0044381 A1, that complexes with calcium, magnesium, barium, strontium, manganese, zinc, copper and iron can be prepared.
However, none of the documents discloses a specific process for the preparation of the iminodisuccinic acid ammonium metal salts according to the invention in which not all ammonium groups are replaced with alkali metals, alkaline earth metals or heavy metals.